To access traditional analog and ISDN subscribers and private branch exchanges, remote switching units (RSU) or access networks (AN) are used. Both represent devices which are external to the exchange and which are linked to the peripheral devices of the exchange via consolidating-type interfaces.
The remote switching units/access networks have mechanisms, formed in accordance with their respective definition, for delivering the subscriber signaling to the peripheral devices of the switching system. At the same time, signaling capabilities are provided for controlling the remote switching unit/access network themselves. For example, these relate to switching through voice channels of the subscriber line or of the branch exchange to voice channels of the interface of the remote switching unit/access network to the peripheral devices of the switching node. The operating state of subscriber line and branch exchange is also adjusted on this path between remote switching unit/access network and the switching system.
As a rule, the subscriber signaling and control information is exchanged in a message-based fashion in the manner predetermined by the interface definition. Typically, the entire interface is PCM-based and certain time slots are reserved for the message-based transmission of subscriber/branch exchange signaling and control information.
An example of a consolidating-type interface between an access network and a local exchange is the V5.2 interface. This provides for ISDN signaling, ISDN packet data to be transmitted on the D channel and message-based analog signaling (PSTN) in time slots of the interface which can be defined according to specifications of the standard. To control the access network, signaling protocols are provided which relate to the switching-through of information through the access network, the adjustment of port and PCM link states and the switching of failed signaling channels to standby.
Although proprietary message-based consolidating-type interfaces can differ significantly from the specifications of the V5.2 standard in terms of the definition, all message-based interfaces to remote switching units/access networks have the common feature that they transmit subscriber signaling information (ISDN signaling messages, ISDN packet data, PSTN messages) and control information for switching-through or for port state adjustment in certain time slots of a TDM-based interface.
The convergence of packet-based networks and TDM networks leads to the problem of how to provide service to traditional analog and ISDN subscribers and private branch exchanges via packet-based transmission methods.
The Internet can be accessed in a highly efficient manner via xDSL links to the subscriber line or via cable networks. This provides sufficient bandwidth in the local loop to be able to handle narrow-band traditional telephony/fax traffic additively via the same access line.
From the point of view of the subscriber, additive connectivity of conventional subscriber terminals and private branch exchanges should thus be possible. In addition, all subscriber features known from the traditional PSTN/ISDN networks should remain available. However, the user data necessary for the switching traffic are transmitted in a packet-based manner in contrast to the situation of a remote switching unit/access network. The same is true of the signaling.
In this situation, the problem arises how to connect traditional subscribers or branch exchanges (i.e. POTS, ISDN BA, ISDN PRI), which are served via subscriber termination devices (e.g. IAD of an xDSL link, set-top box of a cable network) with access to a packet network, in such a manner that all traditional subscriber features (including D-channel packet data) are available, in principle. Furthermore, it should be possible to reuse the HW and SW functions of a traditional exchange, user data streams should be preferably conducted in a packet-based manner or optionally it should be possible to bring them into the exchange by TDM technology and to transmit signaling information and ISDN packet data to the exchange in a packet-based manner.
In the prior art, these traditional subscribers are carried as a part of an access network to solve this problem. This network exhibits centralized components (e.g. IAT) which convert the narrow-band user data stream into TDM technology and provide signaling and user data streams in the form of a V5.2 interface. The user data streams are then processed in a conventional local exchange of the PSTN/ISDN network. Broadband traffic is supplied and forwarded by means of suitable upstream multiplexing devices. The corresponding situations are shown diagrammatically in FIG. 1.
Accordingly, an access network AN is illustrated which supplies information from a plurality of subscriber terminals to the peripheral devices of a local exchange LE. The subscriber termination device provided is a device IAD which supports specific functions of xDSL transmission methods. In a multiplexer DSLAM, narrow-band information is separated from broadband information and the xDSL link is terminated. The voice and signaling information is supplied in an IP-based manner via an ATM network as carrier to a device IAT which forms the head end of the access network AN. The latter supplies the voice and signaling information to a local exchange LE via a V5.2 interface.
The advantages of the concept illustrated there are, on the one hand, the utilization of an existing consolidating-type interface and, on the other hand, the availability of all subscriber features supported by the exchange. The decisive disadvantage, on the other hand, lies in the utilization of TDM technology in an exchange active in the environment of convergent networks. To this is added a deterioration in the voice quality due to the possibly avoidable conversion of the user data stream between TDM technology with codec G.711 and the packet-based transmission by means of a generally compressing-type codec such as e.g. G.723.1.